Yet experiments like these are going forward just the same. In just the past few months, scientists at the University of Wisconsin and the University of Rochester have published data on their human-animal neural chimeras. For the Wisconsin study, researchers injected mice with an immunotoxin to destroy a part of their brains—the hippocampus—that's associated with learning, memory, and spatial reasoning. Then the researchers replaced those damaged cells with cells derived from human embryos. The cells proliferated and the lab chimeras recovered their ability to navigate a water maze.
For the Rochester study, researchers implanted newborn mice with nascent human glial cells, which help support and nourish neurons in the brain. Six months later, the human parts had elbowed out the mouse equivalents, and the animals had enhanced ability to solve a simple maze and learn conditioned cues
These protocols might run afoul of the anti-hybrid laws, and perhaps they should arouse some questions. These chimeric mice may not be human, or even really humanish, but they're certainly one step further down the path to Algernon. It may not be so long before we're faced with some hairy bioethics: What rights should we assign to mice with human brains?
All that has to do with sticking human parts in beasts. Transplants also go the other way: Heart valves and ligaments can be extracted from a pig, stripped of excess porcine cells, and implanted in a person. In the not-too-distant future we may see more substantive forms of xenotransplantation, particularly from pigs that have been genetically engineered to make their parts more amenable to human use. (These animals lack a pig-specific protein that induces a strong immune response from people.) Transplanted pig corneas and pancreatic cells will be next, and we might see hearts and kidneys one day as well.
These procedures haven't generated much angst, except perhaps from orthodox rabbis. Presumably that's because they'd leave us with a fully human brain, and that's where we like to think our deepest personhood resides. Transplanted pig parts also fail to violate a taboo against enhancement: They're therapies, not upgrades—a way to mend a broken part.
What if animal organs, cells, or DNA could make us faster, stronger, or more adept? The Department of Defense has dabbled with the notion of dosing humans with bacteria taken from a pig's digestive tract. This research program, known as "Intestinal Fortitude," could in theory help a solder to digest cellulose or otherwise survive on "non-traditional foodstuffs."
No one has transfected a living human with nonhuman genes in an effort to bestow cheetah-speed or night vision, or tested radioactive spider-bites in a controlled, laboratory setting. "Let's put it this way," says Maxwell Mehlman, a bioethicist at Case Western and author of a book on transhumanism, "we can't even figure out a good way of gene doping using human genes."
One hundred years ago, a doctor named John Brinkley started implanting patients with goat testicles as a way to make them virile. These days, quack physicians promise to invigorate us with deer antlers. The technology may change, but the dream of manimal enhancement never goes away.